Terminating cheapo SCA -> 50Pin SCSI adapters

[uniserver]

yeah i am pretty sure BMOW has this very same issue with his Floppy Emu, The screen goes weird every time plug it into this 512k i have here, The thing works but might have some dirty power output or something.

maybe i should ask em to put a nice volt meter on the screen of his floppy emu??? that would be useful… pop on and you can see right whats/what. make it more useful and more of a diagnostic tool as well!

maybe just a one liner at the top? +12=XX.XX -12=XX.XX +5=XX.XX

. 5 -12V

. 6 +5V

. 7 +12V

. 8 +12V

or go the extra mile and program in limits… and the floppy emu says… HEY YOUR PSU IS NOT GOOD DOOD! FIX IT! hahah that would be Cool!!

 

[max1zzz]

There's a simple test is you think you have dirty power on the 512k, grab some big caps (1000uf or so) and stick them on the voltage rails on the floppyemu, should give it much cleaner power

Had the same issue with a project i built in my electronics class in school a year or so back, microcontroller went crazy every time i tried to power up the board, after a week of literally checking everything on the board i tried putting a 1000uf cap over the power input, and it worked perfectly from then on. Turns out the power brick i was using wasn't so well regulated

 

[max1zzz]

Ok, this seems to work nicely, just 3 SMT components. This also removes the need to add the 10uf electrolytic cap

[attachment=0]photo(21).JPG[/attachment]

And it should cram into the custom PCB's fine aswell

 

[trag]

And there isn't anything wrong with the way i have been doing it, just it relies on the 5v rail being the correct voltage,

Actually, I think there was.

With a voltage regulator in there, as you've switched to, when the signal pins are high (2.85 - 5V) there will be some current flow from the signal pin through the 110 ohm resistor to the regulated voltage area, damping ringing on the high end.

Under your previous configuration, when the signal was high, there could be no current flow to damp the ringing because the diodes acting as the voltage regulator would be reverse biased. Using diodes as a voltage regulator only works if the current flow is always in the same direction and if the difference in voltage between the ends is large enough to forward bias the diodes. Neither of these conditions are always met in this application.

I have not given full consideration to how the capacitor may have contributed to the termination, though, hence my earlier request for an explanation of how it was meant to work. It's possible I've overlooked an aspect which would make it work. But based on what I know so far, I really don't think it should work properly -- using a real voltage regulator is a much more robust solution.

Of course, in many instances, even no termination will still allow a SCSI bus to operate, so the fact that some systems with the old style terminators work is not proof that they're doing their job properly.

Anyway, all that said, I think it's great that you're moving to using a real voltage regulator circuit. Nice work.

 

[max1zzz]

Ahh i see. Well, it worked where no termination didn't, but isn't the correct way to do it

So, is it ok to use a 3.3v reg like the one i used in that picture? Been looking around and they are cheaper and easier to get than the 2.85v variant

 

[trag]

So, is it ok to use a 3.3v reg like the one i used in that picture? Been looking around and they are cheaper and easier to get than the 2.85v variant

I do not know enough to answer this question. The specification says to use 2.85V. I imagine that you could get some ringing on the high signals because 3.3V is a little too high and your highs will be a little underdamped, and your lows will be a little overdamped. With slow original SCSI and for these applications, will it ever make a difference, or will it be "good enough"? I just don't know. There was probably a time when I could have calculated/analyzed it, but those days were about a decade ago....

If you can get 2.85V or an adjustable Vreg without blowing the budget, I'd go that route. I think the LM317 is adjustable, but then you need more components. You might hunt around at National Semiconductors, TI, and Linear Tech (I think someone bought NS a year or two ago) in their power and control or voltage regulation sections and identify a few simple chips that can do the job, then hunt them on Ebay to see the pricing. Also, take a look at questcomp.com. Sometimes they have stuff in stock at lower prices than Ebay. The LT1086 is a good choice, if available in a smaller package than I've seen it in, but I don't know about pricing. LT might have something similar with a lower current rating. Oh, it's worth checking Digikey and Mouser as well. Oh, you're in the UK right? So Farnell?

 

[max1zzz]

i'll just find some 2.85's then, better to do it correctley now than have to change it again further down the line

This is a good price for them : http://www.aliexpress.com/item/Ams1117-2-85-2-85v-1a-power-supply-voltage-stabilizing/771958640.html

I can't seem to find any uk suppliers that do them for a decent price, so it looks like getting them from china is my only choice

 

[uniserver]

thats a good deal man

 

[techknight]

careful they might be fakes. In UK you wanna use farnell.

 

[max1zzz]

Yeah that was my worry, However i think i'm going to take the risk. As the cheapest i can find them is from in the uk is from RS, ~30 pence each (+ shipping +a card handling charge as my order would be under £20)

Edit: the best ones from farnell is : http://uk.farnell.com/rohm/bu2jtd3wg-tr/ldo-fixed-2-85v-0-2a-ssop-5/dp/2342945

However the £20 pretax minimum order limit is a real problem for me

 

[uniserver]

that website seems fairly massive.

I doubt a vendor would risk getting black listed from such a large sales medium...

but then again ... Ya never know... I would say its worth the 8-10 bucks.

I was going to buy 2000 Murata 16v 47uf ceramic caps from there last fall... for like 206.00 free shipping.

sick deal!

I have not found a deal like that this new year.

mouser also sells that brand.

 

[max1zzz]

Yeah i'm gonna take the risk

That happened to me aswell, a few days ago i was looking at 500 10uf 16v ceramics for ~£6, yesterday i went to grab them and they where gone.

 

[trag]

I have bought a few items on Alibaba, and always had a good experience, so far. However, they are a marketplace, not a department store, so who knows how well they police their participants.

At the low currents you're dealing with, even if the Vregs are fakes, as long as there is a Vreg in the package and not just empty packages, you're probably okay. They might be rated for a little lower power or have a slower response time, but as long as they hold 2.85V closely, you're probably okay.

 

[max1zzz]

A slightly modified PCB using the proper voltage regulator setup (this was harder that i thought it would be, had to swap a few parts for their SMT equivs)[attachment=0]Screen Shot 2014-02-05 at 22.11.58.png[/attachment]

I'll order these once i have confirmed the boards i have ordered work